Fundamental Studies On Preparation Of Al-Sc-Zr Alloys By Electrolysis In Molten Salts

Al-Sc-Zr alloy is considered to be an important structural material in aerospace industry, military industry, transportation area and civil application as it possesses the excellent properties such as high strength, high toughness, low density, corrosion resistance, fine weldability, etc. It has attracted much attention in recent year due to the great potential in many advanced applications. Generally, Al-Sc/Zr alloy is prepared by melting Al, Sc and Zr pure metal together in a furnace, thermal reduction in a vacuum furnace and molten salt electrolytic method. Most of the preparation methods are focus on the low value-added master alloys. The contents and structure of the master alloys are far from the applied commercial Al-Sc-Zr alloys. The fluctuation and heterogeneous distribution of Sc/Zr content in master alloys seriously impact on the properties and application of mix-melted Al-Sc-Zr alloy products. What's more, the process of mix-melting method has significant shortcomings such as long flow, high smelting temperature, high production cost. Therefore, this paper will present our recent study on the process of preparing Al-Sc-Zr alloy by electrolysis in molten salt system, where Sc₂O₃ and ZrO₂ were added as raw materials, liquid Al was used as cathode. The purpose of this work was to prepare Al-Sc-Zr alloys approaching to the commercial alloys both in content and structure. The main results of the thesis can be summarized as follows:?1? The geometric and mechanical properties of L1₂-Al₃(Sc_1-xZr_x) ?0<x<0.5? have been systematically evaluated using first principles calculations through VCA framework within the generalized gradient approximation. The calculated thermostability, lattice constants, shear modulus, Young's modulus of L1₂-Al₃(Sc_1-xZr_x) decrease with Zr addition, while the bulk modulus can increase. The ductility of L1₂-Al₃(Sc_1-xZr_x) can be improved with Zr addition estimated by Pugh empirical criterion and Cauchy pressure. The ideal strength of L1₂-Al₃(Sc_1-xZr_x) in [001]. [110] and {111}[110] directions increase linearly with the Zr content x. The strength of the precipitates is relative weak in [110] direction, while Zr addition can increase the ideal strength along this fractured direction. The critical strains on the strain-stress relationship are found to increase, and Zr addition has a positive effect on the ductility in [001] direction than in [110]. In addition, Zr has a positive effect on the stiffness and ductility of the crystals under pressure. The effect of Zr on the ideal strength and ductility of L1₂-Al₃(Sc_1-xZr_x) can be explained due to the increased valence electrons. The changed electron mechanism with Zr addition may strengthen the p-d bonding and d-d bonding within L1₂-Al₃(Sc_1-xZr_x), thus improving its strength and ductility. In order to gain good performance Al-Sc-Zr alloy, L1₂-Al₃(Sc_1-xZr_x) precipitates, in which the ratio of Sc/Zr?1, are necessary in the alloys. These calculation results can provide a theoretical basis for the composition design of Al-Sc-Zr alloys.?2? The behavior of electrochemical reduction of Sc³⁺, Zr⁴⁺ ions and penetration-expansion of the cathode materials were investigated by electrochemical workstation and modified Rapoport tester in sodium cryolite. The results indicate that, deposition of metallic Sc and Zr from the fluorides mixture onto the electrode surface proceeds in a single or two steps with three arid two electrons transferring. The electro-reduction of Zr⁴⁺ takes place on the surface of the cathode at more positive potential than the Sc³⁺. On the other hand, the cathode expansion rate increased with the increase cryolite ratio, potassium. The addition of potassium could aggravate the penetration of electrolyte into carbon cathode. So we choose sodium cryolite ?CR=2.4? without KF addictive as the solvent.?3? Al-Sc-Zr alloys were prepared by molten salt electrolysis in sodium cryolite ?CR=2.4, industrial grade?, where Sc₂O₃ and ZrO₂ were added as raw materials. The results indicate that the preparation process could run smoothly under galvanostatic electrolysis. Al-Sc-Zr alloys with Sc content of 0.18%?0.53% and Zr content of 0.12%?3.27% were prepared by adjustment of the electrolysis condition. The content of some prepared alloys were very close to the commercial alloys. In addition, the effect of cooling rate on microstructure and morphology of the primary precipitates in Al-Sc-Zr alloy were studied. The results show that, the grain size of alloy structure is large at furnace cooling and air cooling condition consisting of a?Al? and L1₂ and D0₂₃ type primary phase. At quick cooling rate ?water cooling?, the D0₂₃ structured primary phase was suppressed and L1₂ type dominated the primary precipitates, which has lower grain size of the structure.?4? The microstructure of Al-Sc-Zr alloys obtained by molten salt electrolysis has been analysis by the optical microscope observation. Image J analysis software and SEM-EDS method. The results show that the average grain size of the alloys decreased with the increased w?Sc?/w?Zr?. The primary particles with regular shape, such as square, triangle and flower, were homogeneous distribution in the alloys. The particle was of layered construction. It was nucleating with Al₃Zr as nucleation and interleaved with the Zr-rich or Sc-rich Al₃?Sc,Zr? layer. The atomic ratio of Sc and Zr in the Al₃?Sc,Zr? grains is corresponding to the content of Sc and Zr in the alloys. The effect of the precipitate with varying proportion of Sc and Zr atoms on the refinement of the microstructure is different. In addition, the first principle calculations show that Al₃?Sc,Zr? particle could serve as heterogeneous nucleation for a?Al?, and it was inclined to nucleate on the Sc/Zr termination layer.?5? The effect of ultrasound on the formation of Al-Sc alloys with molten salt electrolysis in KF-AlF₃ system has been investigated. The results indicate that, Ultrasound can accelerate the electrolytic product transfer into liquid aluminum and improve the current efficiecy. When the ultrasound was introduced in the electrolysis system, a significant increase of the Sc content in the alloy occurred at a low current density. The efficient improvement of ultrasound effect weakened and vanished when electrolysis at a high current density. It will be of great significant in improving process technology for preparing Al alloy by molten salt electrolysis.